Conventionally, in prior art optical scanners, relative motion between the focused image of a light source such as stellar light and a photosensitive detector was accomplished by mechanical scanning methods such as taught by U.S. Pat. No. 3,015,249. This patent teaches a stellar scanner comprising a telescope with a total reflective prism pivoted about the objective lens for focusing the stellar light onto the focal plane. Scanning means provided at the focal plane modulates the image into light pulses synchronous with a reference frequency that is identical to the scanner rotation. The phase relationship between the starlight-pulses and the reference frequency is computed to provide the direction to the star image in relation to a predetemined reference on the telescope. Complex electromechanical means are employed in the invention to determine the ultimate direction to the star image. Other examples of mechanical scanner methods are disclosed in U.S. Pat. No. 2,934,824 and 3,006,236. Generally, the mechanical scanning methods taught by the prior art encompass rotating or angularly deflecting mirrors, optical wedges, translation and/or rotation of the photodetector past the stationary star image, nutating optical fibers and internal electronic scanning image tubes such as the vidicon and image dissector.
The main disadvantages of the mechanical scanner devices are relatively low frequency operation, high power consumption, poor reliability and requirement of optical encoding devices. Mechanical scanners are useful in those applications where the observation period of the light pulses is very short or where weight, size and power are not critical factors. In applications that require low power, small size and very high reliability such as long term missions as the Mars, Venus and/or planetary grand tour probes, the use of mechanical scanning devices based on mechanical motion of some portion of the optical system is precluded.
There are other disadvantages attendant upon mechanical scanner methods. For example, mirror scanning may be accompanied by mirror deformation because of severe environmental conditions which could result in a permanent loss in accuracy. In the case of the rotating optical wedge, the angular deviation of the image raised is usually small and the scan motion is restricted to a circular sweep. Modification of the wedge rotational system for an additional degree of freedom to obtain a linear sweep would lead to a very complicated highly unreliable system. The nutating optical fiber has the disadvantage of limited frequency response, small scan angle and degraded performance when subjected to vibration. Although the vidicon tube and the image dissector are characterized by very high signal-to-noise ratio, derived from the image storage capabilities, they are subject to severe disadvantages. These are high cost, poor reliability, poor radiation resistance, sensitivity to magnetic interference, sensitivity to surface temporal degradation, high power consumption, large size and the requirement of complex signal processing.